1. Field of the Invention
The present invention relates to a semiconductor device and a manufacturing method for the same. In particular, it relates to a semiconductor device using a semiconductor layer doped with impurity atoms having different covalent bond radii.
2. Description of the Related Art
Control of an impurity distribution in a semiconductor layer is extremely important for a semiconductor device. Generally, a thermal diffusion process is used as a technique to form an impurity distribution in a semiconductor layer with high controllability. However, depending on a doping impurity atom, an insufficient concentration of an electrically active impurity atom is provided. Alternatively, the desired impurity distribution is not provided which results in a large diffusion coefficient of the impurity atom. For example, in order to suppress a short channel effect and achieve a high drive current in a metal-oxide-semiconductor field effect transistor (MOSFET) fabricated on a semiconductor substrate such as silicon (Si), it is necessary to form a super steep retrograde channel profile (SSRP), where impurity concentration decreases rapidly toward a gate oxide layer in a channel region of the MOSFET. Otherwise, in a source/drain region, it is required to form a shallow impurity diffusion layer with low resistance.
Indium (In) is used as an acceptor impurity atom in order to form the SSRP of an n-channel MOSFET. However, it is difficult to form a high carrier concentration in a deep channel region because of a low active impurity concentration of In. In addition, in a source/drain region of a p-channel MOSFET, boron (B) is used as an acceptor impurity atom. However, it is very difficult to form a shallow impurity diffusion layer so that a diffusion coefficient of B in a Si crystal is large. Further, because of the low active impurity concentration, In cannot be used as an acceptor impurity atom of the source/drain region of the p-channel MOSFET.
Many proposals for a technique such as “co-doping” which dopes plural impurity atoms together in a semiconductor layer, have been disclosed. According to the co-doping technique, in order to reduce the crystal defect in a semiconductor layer by a vapor phase growth, a method in which phosphorus (P) and arsenic (As) are doped together in a Si layer has been reported (see Japanese Patent Application Nos. 55-028215 and 55-025492). However, a carrier concentration cannot exceed the active impurity concentration of P and As. In addition, in order to suppress diffusion of an acceptor atom, a method to co-dope a constituent semiconductor atom in addition to the acceptor atom has been reported (see Japanese Patent Laid-Open No. 2000-68225). In addition, when doping B into the Si layer, a method to co-dope germanium (Ge) is used. However, it is necessary to dope Ge with a high concentration to change the activation rate and the diffusion for. B.
As described above, in a semiconductor device such as the MOSFET, the doping layer of the high impurity concentration is needed in the channel or the source/drain region. However, it is impossible to achieve a higher carrier concentration exceeding the active impurity concentration of the dopant atom. It is also impossible to suppress the diffusion of the heavily doped impurity atom.